A clinical trials network is an essential component of an overall program to develop and test candidate HIV vaccines, and needs the scientific leadership and practical capacity to take products expeditiously from Phase I studies through efficacy trials. This application brings together participants in the UCSF AIDS Research Institute (ARI) who are leaders n the fields of virology, immunology, epidemiology, and behavioral science to design and conduct HIV vaccine trials. )ver the last decade, we have carried out pre-clinical and efficacy trial preparedness studies, single- and multi-site Phase I - III HIV vaccine trials, and laboratory support f6r all domestic and international HIV Network for Prevention Trials HIVNET) protocols. From this group of ARI investigators, the HVTN can expect expertise and leadership in the design, implementation, and laboratory support of vaccine trials. We work closely with dedicated community members to address issues of importance in the conduct of vaccine trials, and propose in this application to link domestic with international community advisory boards. "Sister" sites will jointly address scientific, operational and ethical issues to help ensure that domestic and international agendas advance cooperatively. We also bring to this application substantial capacity to enroll high risk men who have sex with men (MSM) for inclusion in Phase II, Phase IIB, and Phase III vaccine trials. Data from previous studies demonstrate our success in identifying and enrolling diverse cohorts of high risk MSM in vaccine and other prevention trials; HIV seroincidence among our participants has been consistently in excess of 2 per 100 person1years, after administration of enhanced risk reduction counseling and decreases in reported risk behavior over the course of these trials. These high seroincidence rates are likely the joint result of high HIV seroprevalence in the MSM community (estimated at 30%), and identification f participants with substantial risk behavior. We have expanded our capacity to enroll high-risk MSM by inclusion of experienced sites in other Northern California locations including Santa Clara, Sacramento, Marin, and Sonoma counties. We will work within the HVTN to refine and implement a comprehensive scientific agenda for evaluating HIV vaccines, and bring substantial expertise and capacity to carry out such trials in the San Francisco HIV Vaccine Clinical Trials Unit.